The invention relates to methods and apparatus for detecting an impedance mismatch between a directional coupler and an antenna in a radio frequency (RF) communications system in the presence of externally generated RF signals.
Presently there exist methods and apparatus for detecting impedance mismatch for use in tuning antenna impedance matching circuits. These detecting methods and apparatus are used in RF communication systems where a communications signal is passed through a directional coupler and an impedance matching circuit to an antenna.
The directional coupler allows for the detection of a forward voltage and a reflected voltage, which are combined through quadrature multiplication to give an error signal used to adjust impedance matching. The error signal is directly related to the detected impedance mismatch.
An example of a tuning system in the prior art is described in U.S. Pat. No. 4,493,112 to Bruene which shows a circuit that performs quadrature multiplication of a forward voltage and a reflected voltage sampled from a directional coupler to produce an error signal. The product error signal is applied to an impedance matching circuit to tune the circuit.
The error signal, which is a direct current (d.c.) signal, is produced as a result of the quadrature multiplication and is produced only when the forward and reflected signals, which are multiplied, have components of like frequency, i.e., multiplication of voltages of unlike frequency does not produce a d.c. output signal. Thus, the magnitude of the error signal will be proportional to the sum of the products of the individual voltage components which are of like frequency.
The quadrature multiplication process involves the application of a signal to a first multiplier and a 90.degree. phase shifted version of the same signal to a second multiplier. Use of the output signals from both of the multipliers insures that the error signal will not be zero due to multiplication of signals that are 90.degree. out of phase.
The operational assumption which underlies the prior art is that the magnitude of the error signal is directly proportional to the magnitude of the reflected voltage which in turn corresponds to the degree of impedance mismatch. Because the forward voltage is assumed to contain only signals of the desired frequency, a reduction of the error signal corresponds with a reduction in the reflected voltage, which indicates the degree of impedance matching. Accordingly, impedance matching is enhanced when the error signal is reduced.
However, prior art systems such as the one described in the Bruene patent are disadvantaged by this assumption in that the forward voltage may contain significant components attributable to externally generated RF signal. In actual operation, the forward voltage is made up of several components. Some of the forward voltage components are attributable to the communications signal as assumed. The forward voltage components attributable to the communications signal include attenuated reflections of the forward voltage which occur due to impedance mismatch. These twice reflected components are related to the communications signal and generally have the same frequency as the communications signal.
In addition, there may be significant other components in the forward voltage attributable to signals coming into the system through the antenna from external sources (externally generated RF signals), and to reflections of them in the forward voltage. It is assumed that the components of the forward voltage from external sources have frequencies other than that of the communications signal.
Accordingly, quadrature multiplication of the forward and reflected voltages produces a d.c. output or error signal which is related in part to interference at the desired communications signal frequency and in part to interference from externally generated RF signals not at the desired frequency. Thus there is an undesired d.c. component in the error signal, and tuning to reduce impedance mismatch at the communications signal frequency using an error signal not entirely related to such mismatch is not optimum. The tuning of the impedance matching circuit may further be disadvantaged by the uncontrollable timing and magnitude of the occurrence of externally generated signals, leading to undesirable hunting by the impedance matching circuit.
Embodiments of the present invention improve upon known systems by detecting impedance mismatch in the presence of externally generated RF signals not at the frequency of the communications signal. Impedance mismatch is detected in such a way that the influence of the externally generated RF signals on the error signal is reduced.
One embodiment of the present invention performs quadrature multiplication using a voltage related to the carrier signal (which only has components with the frequency of the desired communications signal) in contrast to the forward voltage (which has components from external sources).
In effect, the use of the carrier signal voltage in the quadrature multiplication enables the detection of impedance mismatch attributable only to interference at the frequency of the communications signal in the presence of externally generated RF signals. Because the carrier signal does not have components attributable to external signals, there will be no d.c. error signal component produced by the quadrature multiplication that corresponds to the external signals. Accordingly, impedance matching that is carried out in response to the error signal produced using the carrier signal, will not be thrown off by error signal components that do not relate to impedance mismatch at the desired communications frequency.
Another embodiment of the invention permits the use of the directional coupler forward voltage, but only after attenuating the signals attributable to external sources present in the forward voltage. The attenuation of externally generated RF signals in the forward voltage, before quadrature multiplication, yields an error signal with a smaller component attributable to the external signals in comparison to the use of forward voltages without attenuation.
It is accordingly an object of the present invention to provide a novel circuit and method of detecting impedance mismatch between a source and an antenna in the presence of external source signals.
It is another object of the present invention to provide a novel circuit and method of detecting impedance mismatch so that the error signal produced by quadrature multiplication is almost exclusively related to the reflected communications signal.
It is also an object of the present invention to provide a novel circuit and method of detecting impedance mismatch so that impedance matching circuit tuning is substantially non-responsive to signals from external sources.